I've driven a D4 Premio (the one with the 3S-FSE D4 VVTi engine) for over 2 years with no issues. The key (so far) is taking care of the car. It's not one of those cars you can 'abuse' by postponing service etc. Also,use the right plugs (I have the numbers somewhere).
I also watch where I fuel. That said,I sometimes wonder whether I get the best out of the car especially in light of @wachiras comments on the kind of fuel available.
I've done 13-15.5km/l on the highway which I find impressive. In town it's just about 10km/l (slightly less when i drive aggressively). (Think like going to a place like Voi and back with some fuel left to spare)
Incidentally the technology is not unique to Toyota - the Mitsubishi GDI's,the VW/SKODA/AUDI FSI is more or less the same tech. Read this wikipedia entry for the technical mumbo jumbo if you are interested -
http://en.wikipedia.org/wiki/GDI_Engine
I don't know much about the D-4D though only that it's a diesel engine, and I doubt it's directly related to the petrol D4 engine.
Here's some stuff I have managed to find online though:
D-4D actually stands for direct Injection 4 Cylinder Common Rail Diesel Engine. Common rail diesels inject a fine mist of fuel into the engine at very high pressure. The result is the same power and refinement as a petrol engine,but with better fuel efficiency and lower emissions. Toyota D-4D engines are not just noteworthy for their smooth,linear torque,though: that's a characteristic of most modern diesel engines. What D-4Ds are noteworthy for is their unusually high power outputs,equivalent to that of many competitors' similarly-sized petrol engines. The D-4D's rev range is also usefully wider than normal,further enhancing the driving experience.
http://www.thaivisa.com/forum/D4d-t80930.html